ReplicatedPG: don't rescan the local collection if we can avoid it

Signed-off-by: Samuel Just <sam.just@inktank.com>
PGBackend,ReplicatedBackend: add interfaces for scanning the pg

This will be important since the erasure coded pg will have a different
on-disk format than the replicated backend.

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG.cc: remove leading empty space

Signed-off-by: Samuel Just <sam.just@inktank.com>
PG,ReplicatedPG: expose PGBackend to PG

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG,PG: adapt collection_list* users to PGBackend interface

Signed-off-by: Samuel Just <sam.just@inktank.com>
ReplicatedPG,PG: use PGBackend methods for getattr

Signed-off-by: Samuel Just <sam.just@inktank.com>